1. Field of the Invention
The present invention relates to an apparatus for and a method of performing a speech rate modification in which only the time duration of speech is changed without altering the fundamental frequency components of the speech signal.
2. Description of the Prior Art
Heretofore, in order to perform a speeded-up listening or a slowed-down listening of speech signals recorded on audio tapes or the like, speech rate modification systems have been utilized.
Speech rate modification apparatus of prior art have included U.S. Pat. No. 3,786,195, to Schiffman et al., "Variable Delay Line Signal Processor for Sound Reproduction". This speech rate modification apparatus is comprised of a variable delay line, a ramp level and amplitude changer, a blanking circuit, a blanking pulse generator, and a ramp pulse-train generator.
The operation of the speech rate modification apparatus described above is elucidated below.
The input signal is first written into the variable delay line. Next, the ramp pulse-train generator controls the ramp level and amplitude changer and the blanking pulse generator corresponding to a time-scale modification ratio. Then the level and amplitude changer performs a read-out operation of signals from the variable delay line with a speed which is different from the speed used at the time of write-in operation and depends on the time-axis modification ratio. That is, when the reproduction rate of a tape is increased, the read-out operation of the data from a memory is made slower than the write-in operation to the memory in order to restore raised tone (frequencies) to normal levels; whereas when the reproduction rate of a tape is decreased, the read-out operation of the data from the memory is made faster than the write-in operation of the data to the memory in order to restore lowered tones to normal tones. Then, on discontinuous parts between respective speech blocks, the blanking circuit applies a muting action on the output of the variable delay line.
In the conventional constitution as has been described above, however, when increasing the rate, degradations in the recognizability of consonants necessarily occur owing to the practice of thinning data which is necessary for increasing the rate. And because of the above-mentioned muting, signal amplitude becomes discontinuous, causing the problem that only a speech voice having a poor naturalness can be obtained.
Although there is other means using detection of pitch period, apart from the above-mentioned conventional speech rate modification apparatus, such pitch detection methods can not be applied for the case that background music or noise superimposes on speech to be processed because the extraction of pitch is difficult in such case. Hence the above-mentioned method cannot be considered very suitable.